Subtleties of climb milling

[homebrewed]

This is a story about something I encountered during a milling job on my Sieg SX2 mill.

The issue of climb milling can be tricky. I once encountered something I call "deferred climb milling" while milling a slot in a steel plate. I had the plate installed so the depth of the slot was in the Z axis, so Z was fixed and I was moving X and Y. So the side of the end mill was doing the cutting. The procedure was: after finishing one pass on Y, move Y back so the next pass would be a conventional milling operation. Step X over by the DOC my mill/EM could handle, then do another pass. Here's the deal. If I had moved X while the end mill was in contact with the work, that would have been a climb milling step, so there was some backlash in X waiting to be yanked forward by the cutting forces. So when I brought the end mill into the work to start the next cut, the backlash IN X came into action, increasing the DOC enough that the end mill grabbed the work and stalled the motor in an instant. Fast enough that it unscrewed the collet nut holding the end mill!! What did I do wrong? I failed to lock the X axis. I'm fortunate the end mill didn't break, or throw the work across the room.

Since that time I have tweaked/modified my mill to reduce the backlash in X and Y, enough to permit light climb milling in steel -- but I still make sure to lock all the undriven axes when I'm milling, Mill. Unlock. Step. Lock. Repeat....

 

[benmychree]

Indeed, that is how it is done!

 

[DavidR8]

Thanks for posting this. For my learning as a newbie, I'm wondering how there was backlash in the X-axis if you moved the X-axis to make the next pass.
I apologize if this should be completely obvious to me...

 

[jwmelvin]

I'm wondering how there was backlash in the X-axis...

The x-screw pulled the work closer to the cutter. Backlash allows the table some ability to move the work even closer to the cutter.

 

[DavidR8]

The x-screw pulled the work closer to the cutter. Backlash allows the table some ability to move the work even closer to the cutter.

Gotcha, I think I understand now.
Thanks for the explanation.

 

[homebrewed]

I know it seems counterintuitive, but when the nut+feed screw move the table, the clearance between the nut and feed screw is all on one side of the threads. That is the side that will then provide the backlash when you attempt to do a climb mill. And, to make things worse, if you continue to feed while climb milling, the backlash occurs over and over again so you don't get just one nasty jump, it happens every time the cutter is pushed further into the work.

If you have a manual mill try this experiment: Using the handwheel, move the table a noticeable amount to the left. Now with your hand push on the table to the left. If you have much backlash you will feel the table move some more (if you have a DRO you will see the actual amount of backlash present). That's the backlash you need to think about when you're climb milling.

There's a limit to how much you can reduce backlash in a conventional nut/screw drive because frictional forces will make it difficult to move the axes, and also will increase the wear on them. Ball screws have rolling contact points, which have low friction even with low-to-zero backlash so that's why they are used on CNC machines. It might seem like a no-brainer then to put ball screws on your manual machine, but their friction is so low that they can move under the machining forces -- they won't hold their position. And if you're climb milling that isn't a good thing -- the table could self-feed, despite your hand on the crank. Sounds bad to me.....

 

[Shootymacshootface]

My mill has quite a bit of backlash on both x and y axies. I usually put some tension on the gibbs with the table locks to climb mill.

 

[Tozguy]

Just checking, you were conventional milling in one direction, then move the end mill over and climb mill back in the other direction?

 

[Nutfarmer]

It's best to avoid climb cutting on manual machines. For wood workers it's like trying to feed a board back wards into a table saw. It will catch the work piece and try and throw it. You might git away with it on a heavy machine with a light cut,but it is dangers to your self and others around you. Eventually it will catch up with you and cause damaged parts and or injury.

 

[projectnut]

Climb milling may not be an acceptable practice on less stable machines, but it is done on a daily basis on commercial and/or industrial quality machines. I climb mill on almost a daily basis on my Bridgeport Series I machine. I make sure there is no excess backlash in both the X and Y axis to begin with. I recently adjusted the backlash in the X feed nuts to .005" and the Y feed nuts to .007". I also adjusted the X and Y gibs to remove any side play.

When milling in the X plane I lock the Y axis and put a slight drag on the X axis using the table brake. When climb milling in the Y plane I lock the X axis and put a slight drag on the Y axis using that table brake. If you can get all the play out of the table climb milling will produce a far finer finish.